1. Field of the Invention
The present invention relates to a high efficiency flyback converter with synchronous rectifying to simplify the circuit design, and improve the power efficiency by using a conducting resistor of a synchronous switch to detect the secondary current of the transformer, and by using a logic circuit to control the duty cycle of the synchronous switch.
2. Background of the Invention
In modern times, the requirement of power supply is becoming more and more important since the electronic predictors have grown quickly. Herein, the techniques for the switching power supply has achieved more progress than the traditional power supply. Operating in coordination with the variation of the circuit topology, the switching power supply has been an essential technique in the field of the computer and electronic equipment power source. The switching power supply is a popular device because it provides the power source with the stability, compactness, efficiency and lower cost for computer, rather than the traditional linear transformer driver.
In the switching power supply area, the synchronous rectifying method has been in use for many years. Generally, the synchronous rectifying method is almost applied in the forward converter or the resonant converter. FIG. 1 is the prior art U.S. Pat. No. 5,991,171, and uses the synchronous rectifying technique. The synchronous rectifier control 122 is connected in the secondary circuit of a forward DC-to-DC converter, not in a flyback converter.
FIG. 2 is the prior art U.S. Pat. No. 5,991,172, and presents a flyback converter reducing the turn-on switching losses in a single stage by using a zero-crossing detector device to control the SW operation. Obviously, FIG. 2 does not use the synchronous rectifying technique in the flyback converter. The main disadvantage of U.S. Pat. No. 5,991,172 is that a more complex circuit designed to drive the SW. On the other hand, the zero-crossing detector device equipped for the primary circuit and the feedback signal was the divided voltage. The present invention would be very different from FIG. 2 by controlling the current feedback level and detecting the secondary current with synchronous rectifier in the flyback converter.
As mentioned above, it is obvious that the synchronous rectifier has never been applied to the flyback converter. The reason is that the characteristic of the flyback circuit is often interrupted by the parasitic capacitor and leaking inductor operation to make the current switching waveform more complex. The parasitic capacitor interruption hardly results in getting the synchronous control signal, so there is no use of the synchronous rectifier in the flyback converter. Further, some producers use much more complex controlling methods to control the switch for good performance. But the flyback switching power supply is used extensively for the consideration of low price and simple circuit design. We should not use the complex controlling methods in the flyback converter. This invention provides the flyback converter with a low cost and simplified circuit design.
The present invention relates to a high efficiency flyback converter with a synchronous rectifier by using an equivalent-conducting resistor to be a current sensor. The equivalent-conducting resistor is coupled with a synchronous rectifier to detect the secondary current of the transformer. Further, this invention applies a logic circuit coupled with the synchronous rectifier to control the pulse width and duty cycle of the driving pulse of synchronous switch to improve the efficiency of the flyback converter. The synchronous rectifier reduces the component stress of the diode coupled with the synchronous switch, and the pulse width of the output signal from the synchronous rectifier makes an output diode turn on synchronously with the synchronous switch.
The primary object of this invention is to provide a flyback converter that improves the power supply under low switching loss.
Another object of this invention is to provide a flyback converter that avoids the secondary current flow back.
Another object of this invention is to provide a flyback converter that simplifies the circuit design to make mass production much easier, so the circuit production can have cost lower than the traditional flyback converter.
Another object of this invention is to provide a flyback converter that can be operated in good performance for both continuous or discontinuous conducting modes.
Another object of this invention is to provide a flyback converter that reduces the component stress of the output diode.
In order to achieve the purposes described above, the flyback converter with synchronous rectifying in this invention comprises a power source which provides the power for the flyback converter. The power source is coupled to a flyback switch circuit for switching then outputting a high frequency pulse to a transformer. The transformer has a primary coil coupled to the flyback switch circuit to receive the high frequency pulse, and has two secondary coilsxe2x80x94one a master source, and the other a sub-source. The sub-source is connected to a synchronous rectifier via a diode D1 and a fast charging/discharging path, then the diode D1 and the synchronous rectifier are coupled to a logic circuit to detect the output signal of the synchronous rectifier. Further, the logic circuit is coupled with a buffer to control the sequence driving pulse width and duty cycle for the synchronous switch. The synchronous switch is coupled with an output diode in parallel and is turned on simultaneously with the output diode. The output diode is placed between the master source and a load end, and the load further connects with an output capacitor in parallel.
One of the main distinguishing features in this invention is that there is no use of any current sensor. We apply an equivalent-conducting resistor of the synchronous switch to detect the load current then feedback a voltage to the synchronous rectifier for comparing with a preset voltage level to further regulate the driving pulse for the synchronous switch. Since the main power loop has no inductance element (as a current sensor), this invention makes the power supply have good performance.
In the preferred embodiment, the equivalent-conducting resistor detects the load current to make the synchronous rectifier, the logic circuit and the buffer output a regulated driving pulse when the output diode is on. The synchronous switch turns on with the output diode simultaneously to bypass the current direction of the output diode to reduce the component stress and switching loss of the synchronous switch.